1. Field of the Invention
The present invention relates to solvent-containing compositions based on polychloroprene, to solvent-containing zinc oxide dispersions which are stable during storage, to processes for their production, to their use in solvent-containing polychloroprene formulations and to their use as additives in contact adhesives for inorganic or organic substrates.
2. Description of the Prior Art
Polychloroprene is a highly important raw material today as a component for making rubber adhesives, since contact adhesives can be produced therefrom.
These contact adhesives can be applied using simple devices, such as for example brushes, knives or rollers and they adhere excellently to many materials on whose rough surfaces they become firmly anchored. An even greater reason for the widespread use of contact adhesives is, however, their ability to form adhesive layers which, after a brief drying period, become apparently dry to the touch and produce bonds of immediately detectable strength on being joined with slight pressure during their “contact adhesion period,” “tack time” or “open time”. The immediate onset of crystallization of the polychloroprene allows the bonds to “set” quickly by a rapid increase in strength until the end point is reached.
A major disadvantage of these solvent-containing contact adhesives is the unsatisfactory storage stability of the formulations, which is manifested by phase separation caused by the following factors:    1. Dispersed stabilizers, such as magnesium dioxide and zinc oxide, which are present in dispersed form in the adhesive formulation tend to settle out of the dispersion and form sediments during the storage of the adhesive.    2. In addition, phase separation or so-called “phasing” can occur, i.e. a resin/metal oxide mixture can be flocculated from the formulation during storage.
The addition of magnesium oxide and zinc oxide to adhesives based on polychloroprene is known from the prior art. The presence of these metal oxides has an advantageous effect on the storage stability of the adhesives and on the stability of the adhesive bonds. This effect is based on the fact that polychloroprene has a tendency to cleave off small quantities of hydrogen chloride during storage, for which the metal oxides serve as acceptors.
The addition of about 4% each of magnesium oxide and zinc oxide, based on the rubber content, has proven to be advantageous.
Instead of the oxides, the carbonates of these metals are occasionally also used; zinc carbonate has a more transparent appearance in the adhesive than zinc oxide.
In the past, polychloroprene was for example masticated prior to the production of the adhesive, the oxides being incorporated using a roller or an internal mixer. If non-masticated polychloroprene was used in addition to masticated polychloroprene, the oxides were added to the masticated component. It was also possible to produce polychloroprene/metal oxide mixtures having a high metal oxide content (the masterbatch) and to then add this masterbatch to the adhesive mixtures in the required quantities. For reasons of cost this method is however hardly ever used today.
Currently only non-masticated polychloroprene is used, the oxides and carbonates usually being pasted beforehand in machines commonly used in the coatings industry.
The production of storable polychloroprene adhesives which are resistant to sedimentation and phasing is not possible using the abovementioned methods, particularly in the case of low-viscosity adhesive formulations. It is basically possible to improve their resistance to sedimentation by increasing their viscosity. This does however have the disadvantage that the compositions are more difficult to apply.
The resistance to the cleavage of HCl in the prior art polychloroprene adhesives also leaves room for improvement.
The addition of resins to polychloroprene adhesives is also known. The use of heat-reactive phenol or alkylphenol resins for the production of polychloroprene adhesives allows an improvement in the heat resistance of the adhesive bonds, since these resins form a high-melting complex compound with magnesium oxide which is readily soluble in organic solvents.
The complex formation between the metal oxide and the alkylphenol resin takes place particularly rapidly with MgO in toluene as the solvent, the reaction being considerably accelerated by the use of a small quantity of water (about 1%, based on the resin). Using this method, soluble complexes are formed in organic solvents and modified MgO is formed on the surface, so that MgO can no longer form a sediment. A description of this complex formation can be found in: R. Garrett, R. D. Lawrence, Adhesion Jg. 10, 1966, page 296.
This MgO/resin reaction can however only be applied to a small extent to ZnO. ZnO is known to be essentially inactive in the complex-forming reaction with the alkylphenol resin, cf: R. Jordan and R. Hinterwaldner, “Klebharze” (Adhesive resins), page 124 (Hinterwaldner Publishers, Munich, 1994).
The present invention was therefore based on the problem of providing polychloroprene adhesive compositions having improved stability, and in particular improved resistance to sedimentation, improved resistance to phasing and improved resistance to the cleavage of HCl. The invention is also based on the problem of making the ZnO used as a so-called anti-ageing agent also capable of forming complexes with phenol or alkylphenol resins so that highly heat-resistant adhesive formulations can be obtained using ZnO.